1. Field of the Invention
The present invention generally relates to a tubing drain valve suitable for use in a borehole and a method for operating the disclosed valve. The present invention is particularly useul in a production string where it provides a convenient apparatus and a method for draining production fluid from the string in order to facilitate routine activities, e.g., removal of the downhole pump, injection of chemicals into the producing formation, injection of hot fluid to dissolve paraffin deposited in the production tubing and the like. More particularly, the present invention relates to a tubing drain valve actuated to its open position by application of a force developed across a movable piston by the pressure in the interior of the production string coupled with positive closing by mechanical means.
2. Description of the Background
It is often desirble to have a drain valve located in the wall of a borehole tubing string. It is particularly desirable to have a tubing drain valve located in a production string for a variety of reasons. For example, the efficiency of production strings is often decreased by the deposit of paraffin, wax and similar substances on the interior of the string as the string rises through lower temperature zones from a deep, high temperature production zone. It is necessary to remove these deposits in order to maintain the efficiency of the production string. Present methods for removing such deposits employ hot water or stream which is generally forced down the annulus between the production string and the borehole casing. The hot water or steam enters the production string through the downhole pump and returns to the surface through the string where the elevated fluid temperature slowly dissolves the deposits. Because of the long and circuitous path to the zone of interest, fluid reaching the zone of interest is often at a temperature significantly lower than the injected fluid. It is often desirble to inject the producing zone with a variety of chemicals to increase or aid production. These chemicals also are injected by the previously described method for injecting steam or hot water. These methods suffer from the disadvantage that excessive quantities of fluids and chemicals are required because the injection is indirect through the annulus. Alternatively, dedicated, small diameter injection tubing is employed to directly inject chemicals into the production zone.
It is often necessary to pull the production pump for routine maintenance, repair, replacement and the like. Many presently employed systems require that the production string be pulled in order to pull the production pump. This is necessary because the production pump will not pass through the interior of the production string due to size differential or obstructions in the string. Accordingly, the pulling of the production pump is quite time consuming and expensive. Even where the pump will pass through the production string, present systems for pulling the pump must overcome the vast weight of the production fluid in the string. This weight is substantial and reaches many tons in deep wells where the fluid column is many thousands of feet or even several miles in height.
In an attempt to solve some of the above problems, others have proposed a variety of valves suitable for use in a borehole production string. For example, valves which open when the pressure within the tubing exceeds a pre-determined pressure have been proposed. Such valves permit fluid communication between the interior of the production string and the annulus at the valve location. These valves are useful for draining the production fluid above the valve location and for injecting chemicals into the borehole at the valve location only if the valve remains open after its initial opening. It is still necessary to pull the production string in order to close these valves. Alternatively, other check valves, e.g., spring loaded valves, operable above a pre-determined pressure permit injection of fluids into a borehole at the valve location, but fail to provide a means for draining fluid from the production string.
Accordingly, there has been a long felt but unfulfilled need within the industry for a tubing drain valve which is both opened and closed conveniently and reliably from the surface. This valve must also be easily maintained in the open position in order to permit drainage of the production string. Further, this valve must be positively closable from the surface. Finally, this valve should not provide any obstruction in the interior of the tubing string to the removal of the production pump.